Field of the Invention
The present invention concerns: a method to operate a magnetic resonance device with a magnetic resonance sequence—in particular a PETRA (Pointwise Encoding Time Reduction with Radial Acquisition) sequence—radially scanned for an image acquisition in a first region of k-space that does not include the center of k-space, with an excitation pulse that is radiated as the full strength of the at least two phase coding gradients is reached, and in which k-space is scanned in a Cartesian manner—in particular by single point imaging—in a second, remaining region of k-space other than the first region. The invention concerns a magnetic resonance device implements such a method.
Description of the Prior Art
Magnetic resonance sequences in which extremely short (“ultrashort”) echo times are used offer new fields of use in magnetic resonance imaging. Materials can be made visible with ultrashort echo times—for example bones, ligaments, tendons or teeth—that would not be measurable with conventional sequences (for example in echo sequences or gradient echo sequences) due to their rapidly decaying magnetic resonance signal. Fields of application are therefore, for example, orthopedics, dental or skeletal imaging, and magnetic resonance positron emission tomography attenuation correction.
In the prior art, various magnetic resonance sequences have been developed that have such ultrashort echo times, for example echo times TE<500 μs.
One example of such a magnetic resonance sequence is the radial UTE (ultrashort echo time) sequence, for example as described in an article by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: clinical applications in human brain and skeletal muscle”, Magn. Reson. Med. 2007; 57; Pages 74-81. After a wait time after a non-slice-selective or slice-selective excitation, the gradients are ramped up and begun simultaneously with the data acquisition, wherein the k-space trajectory scanned in such a manner proceeds radially outwardly from the k-space center after an excitation. Before the image data are determined by means of Fourier transformation from the raw data acquired in k-space, the latter must initially be converted into a Cartesian k-space grid (for example via regridding).
An additional known approach for ultrashort echo times <500 μs is to scan k-space in points in that the “free induction decay” (FID) is detected. Such methods are typically designated as single point imaging since essentially only one raw data point in k-space is detected per radio-frequency excitation. The RASP (“rapid single point imaging”) method is an example for such a single point imaging, which is described in an article by O. Heid and M. Deimling, “Rapid Single Point (RASP) Imaging”, SMR, 3rd annual meeting, Page 684, 1995, for example. At a fixed point in time after the radio-frequency excitation at the “echo time” TE, a raw data point in k-space is read out whose phase has been coded by gradients. The gradient strength, together with the echo time, consequently thereby determines the point that is read out. The gradients are changed by means of the magnetic resonance system for each raw data point or, respectively, measurement point, and k-space is thus scanned point by point.
The two presented variants—thus UTE sequences and single point imaging—both have disadvantages, in particular that the methods take a very long measurement time.
In this regard, a magnetic resonance sequence has been proposed that combines both approaches into a more time-effective method, known as the PETRA sequence (“Pointwise Encoding Time Reduction with Radial Acquisition”). The PETRA sequence is described in, for example, an article by David. M. Grodzki et al., “Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA)”, Magnetic Resonance in Medicine 67; Pages 510-518, 2012, and in DE 10 2010 041 446 A1, which is herewith incorporated by reference into the disclosure content of this Specification. In a PETRA magnetic resonance sequence, k-space corresponding to the imaging region is read out according to the following steps:
a) switching (activating) at least two phase coding gradients in a respective spatial direction by means of a gradient system of the magnetic resonance device,
b) after the switched phase coding gradients have reached the full strength, radiating a non-slice-selective radio-frequency excitation pulse by means of a radio-frequency transmission/reception device of the magnetic resonance device,
c) after a time t1 after the last radiated excitation pulse, acquiring echo signals by means of the radio-frequency transmission/reception device and entering these signals as raw data in k-space along a radial k-space trajectory predetermined by the strength of the phase coding gradients,
d) repeating Steps a) through c) with different phase coding gradients until k-space corresponding to the imaging area is read out (filled) along radial k-space trajectories, in a first region depending on time t1, and
e) reading out (filling) k-space corresponding to the imaging area that is not covered by the first region of k-space, and that includes at least the k-space center in a different manner than described by Steps a) through d).
One of the basic ideas of the PETRA sequence is to already switch the phase coding gradient fields before the excitation pulse and to wait until these gradient fields have reached their full strength, such that the echo time—thus the time that lies between the excitation via a radio-frequency excitation pulse and the start of the acquisition of the measurement data—can be reduced in the totality of k-space to be scanned radially in comparison to a UTE sequence. However, a region around the center of k-space cannot be read out in this way, such that it is proposed to read out this region in a Cartesian manner, in particular by means of a single point imaging method (for example RASP).
The sequence parameters describing the specific magnetic resonance sequence to be executed are thereby largely freely selected by the user. It is extremely complicated, however, to achieve short overall measurement times by an appropriately devised selection of parameters.